1. Technical Field
The present invention relates to specialized metallurgical processes and more particularly to plasma arc cold hearth refining (PACHR) and electron beam cold hearth refining (EBCHR) of titanium or other metals and alloys thereof.
2. Background Information
The prior art discloses a number of processes for the plasma arc cold hearth refining (PACHR) and electron beam cold hearth refining (EBCHR) of titanium and other metals and alloys thereof.
U.S. Pat. No. 5,224,534 to Shimizu, et al., for example, discloses a method of producing a titanium or other metal or titanium or other alloy material by EBCHR which comprises melting the said metallic material and casting a meltable electrode, characterized in that the electrode produced by EBCHR is made by enveloping the said metallic material melted with an enclosure formed from a metallic material having a higher thermal conductivity than said particular metal. The evaporation loss of the alloy element of the said particular metal is compensated by adjusting the input chemistry of the solid particular metal. Titanium sponge or titanium scrap may be produced into a slab with a square cross section and then directly rolling the slab without subjecting the slab to forging before the rolling.
U.S. Pat. No. 6,019,812 to Volas, et al. discloses a PACHR process which provides an ingot of improved properties and including a PACHR furnace operated inside a chamber containing an inert gas, such as helium, 1.1 atm pressure levels. Raw material metals for a desired titanium or titanium alloy composition are supplied to a melting hearth located inside the chamber and heated by a plasma torch which utilizes an inert gas. The plasma arc melts the raw material metal thereby forming a molten pool of metal that is directed to at least one refining hearth. Plasma torches located in the refining hearths maintain the metal in a molten state as it passes through the cold hearth to allow impurities present in the metal to be refined therefrom. After passing through the refining hearths, the molten metal is poured into an ingot mold while still under 1.1 atm inert gas pressure. The molten material is then allowed to cool and solidify into a continuously cast ingot. The thus formed ingot is then subjected to hot working and fabrication operations.
In conventional plasma arc cold hearth refining (PACHR) and electron beam cold hearth refining (EBCHR) of metals such as titanium alloys and superalloys and other metals and their alloys, a water cooled copper hearth is supplied with raw materials in the forms of loose lumps and pieces or premelted fabricated solid bars. This material is melted and refined by plasma arc or electron beam. A solid skull will form when molten metal contacts with the bottom and side wall surfaces of the water cooled copper hearth. A molten metal pool will then form on top of the solid skull. The refined molten metal is poured from the hearth into a cylindrical or rectangular mold to form a continuously cast cylindrical ingot or rectangular slab.
The use of a water cooled copper hearth in a conventional cold hearth furnace (PACHR or EBCHR) has a number of limitations.
One such limitation is that the water cooled hearth removes a significant amount of heat from the molten metal. As a result, high power input from plasma (PACHR) or electron beam (EBCHR) is needed to maintain a desired melting rate and molten metal superheat. Consequently, the thermal efficiency of many prior art systems is low.
Another disadvantage of the prior art methods is that it is necessary to control the heat transfer rate at the bottom and sidewall surfaces of the solid skull. In practice it is found that it is difficult and expensive to effect such control of the heat transfer rate at the bottom and side wall surfaces of the solid skull.
Another disadvantage of the prior art methods is that the water cooled copper hearth, which is used in such methods, is a complex and expensive equipment.
Another disadvantage of the prior art methods is that during operation, the water cooled copper hearth experiences very high temperature gradient which results in high level of thermal stresses. Consequently, the hearth may crack and need expensive repair work. In addition, furnace downtime will also significantly reduce the metal throughput rate.
A still further disadvantage of the method of the prior art is that the setup and exchange of the water cooled copper hearth is a time-consuming work, which reduces overall productivity of the furnace.
It is an object of the present invention for titanium or other metals or their alloys to provide a simple and inexpensive means of controlling heat transfer rates in a plasma arc cold hearth refining (PACHR) or electron beam cold hearth refining (EBCHR) and allows for simple and inexpensive means of water cooling associated apparatus.
It is another object of the present invention to provide a method of plasma arc cold hearth refining (PACHR) and electron beam cold hearth refining (EBCHR) of titanium and other metals or their alloys which substantially avoid high levels of thermal stress and resulting cracking in associated apparatus.
It is still a further object of the present invention to provide a method for plasma arc cold hearth refining (PACHR) and electron beam cold hearth refining (EBCHR) of titanium and other metals and their alloys which can be accomplished with apparatus which is easily inexpensively and quickly set up and assembled to allow practice of the method.
The present invention comprises a method of hearthless block melting (HLBM) and an apparatus for accomplishing this method.
First a solid metal block having an upper processing surface and a base surface is provided which consists essentially of titanium or other metal or alloy which is to be processed. A plasma arc or electron beam is then used to form a pool of molten metal on the upper processing surface of the metal block. The titanium or other metal or alloy to be processed is then added to the pool of molten metal and is melted. The titanium or other metal or alloy melted in this way is then removed from the pool of molten metal and is poured into an ingot mold to form a cylindrical ingot or rectangular slab.
HLBM uses a solid metal block as the molten metal container. The chemical composition of the block is similar to the ingot/slab to be produced. The equipment that is used to replace the water cooled copper hearth includes a water cooled copper base plate, a reusable block sitting on the base plate, and a water cooled copper pour-lip attached to the block. At the start of the operation, the block is first melted by the plasma arc or electron beam to form a molten pool. The raw material is then added at the one end of the block without the pour-lip. The overflow molten metal is poured into the ingot casting mold through the attached pour-lip. The shape of the block is not limited to rectangular. It can be xe2x80x9cCxe2x80x9d shaped, xe2x80x9cTxe2x80x9d shaped, xe2x80x9cLxe2x80x9d shaped, or small ended rectangular or hexagonal shaped. There is no limitation to the number of plasma torches or electron beam guns to be used for the furnace.
The heat transfer rate between the bottom of the block and the base plate can be reduced to maintain a deeper and bigger molten pool in the block. The block bottom to base plate heat transfer rate can be reduced by either insulating the block bottom or machining out a certain groove pattern at the block bottom to reduce the contacting area between the block bottom surface and the base plate. The insulating material can be any metallic as well as non-metallic foil, sheet, plate, and block. The total surface area of the machined groove pattern can be adjusted to change the block/base plate interface heat transfer rate. For plasma arc cold hearth refining (PACHR), helium gas jet can be introduced to selectively cool the block side walls and prevent molten metal flow out from block side walls . A metal shield guide can be used to protect the helium gas pipeline from the plasma or electron beam heat or overflow molten metal. The block can be clamped with the base plate to maintain a close contact and consistent heat transfer rate between the block bottom.surface and the base plate. At the start of the operation, a solid block of metal with the similar chemical composition of the molten metal to be produced is put into the pour-lip. During the melting operation, the top portion of the solid block will be melted away to allow the molten metal to flow through. The bottom portion of the block will stay solid to prevent molten metal having a direct contact with the water cooled copper base plate and losing superheat.